The invention is directed to a resistive memory device comprising a plurality of memory cells, and a related method, a related control unit, a related memory cell, and a related computer program product.
Nanoscale memory devices, whose resistance depends on the history of the electric signals applied, could become critical building blocks in new computing paradigms, such as brain-inspired computing and memcomputing. However, there are key challenges to overcome, such as the high programming power required, noise and resistance drift.
One promising example for resistive memory devices are phase-change memory (PCM) devices. PCM is a non-volatile solid-state memory technology that exploits the reversible, thermally-assisted switching of phase-change materials, in particular chalcogenide compounds such as GST (Germanium-Antimony-Tellurium), between states with different electrical resistance. The fundamental storage unit (the “cell”) can be programmed into a number of different states, or levels, which exhibit different resistance characteristics. The s programmable cell-states can be used to represent different data values, permitting storage of information.
In single-level PCM devices, each cell can be set to one of s=2 states, a “SET” state and a “RESET” state, permitting storage of one bit per cell. In the RESET state, which corresponds to an amorphous state of the phase-change material, the electrical resistance of the cell is very high. By heating to a temperature above its crystallization point and then cooling, the phase-change material can be transformed into a low-resistance, fully-crystalline state. This low-resistance state provides the SET state of the cell. If the cell is then heated to a high temperature, above the melting point of the phase-change material, the material reverts to the fully-amorphous RESET state if rapidly cooled afterwards. In multilevel PCM devices, the cell can be set to s>2 programmable states permitting storage of more than one bit per cell. The different programmable states correspond to different relative proportions of the amorphous and crystalline phases within the volume of phase-change material. In particular, in addition to the two states used for single-level operation, multilevel cells exploit intermediate states in which the cell contains different volumes of the amorphous phase within the otherwise crystalline PCM material. Since the two material phases exhibit a large resistance contrast, varying the size of the amorphous phase within the overall cell volume produces a corresponding variation in cell resistance.
Reading and writing of data in PCM cells is achieved by applying appropriate voltages to the phase-change material via a pair of electrodes associated with each cell. In a write operation, the resulting programming signal causes Joule heating of the phase-change material to an appropriate temperature to induce the desired cell-state on cooling. Reading of PCM cells is performed using cell resistance as a metric for cell-state. An applied read voltage causes current to flow through the cell, this current being dependent on the resistance of the cell. Measurement of the cell current therefore provides an indication of the programmed cell state. A sufficiently low read voltage is used for this resistance metric to ensure that application of the read voltage does not disturb the programmed cell state. Cell state detection can then be performed by comparing the resistance metric with predefined reference levels for the s programmable cell-states.
In spite of the success of PCM technology, reducing the RESET current and increasing the cycling endurance will have significant ramifications on the application space of this technology. The RESET current typically scales inversely with the volume of phase change material that is switched. Cycling endurance can be improved if one could avoid elemental segregation and stoichiometric variations that arise during the operation of the device. Another key challenge is that of resistance drift and noise that limits the number of resistance state one can reliably store and retrieve from the memory device.
A document by Wabe W. Koelmans, Abu Sebastian, Vara Prasad Jonnalagadda, Daniel Krebs, Laurent Dellmann & Evangelos Eleftheriou, Nature Communications, 6, 2015, Article number: 8181, introduces the concept of a projected memory device, whose distinguishing feature is that an electrically conductive segment in parallel with the phase change material can reduce the effect of resistance drift and noise in the phase change material's amorphous phase on cell read operations.
Accordingly, there is a need for further improvements of resistive memory devices.